docs: def numRegisteredForRoleName(event, roleName):
      '''
      This tag allows one to access the number of registrations
      for any dance role using only the role's name.
      '''
      if not isinstance(event, Event):
          return None
  
      try:
          role = DanceRole.objects.get(name=roleName)
      except ObjectDoesNotExist:
          return None
  
      return event.numRegisteredForRole(role)